Field of the Invention
The invention relates in general to a touch sensing circuit, and more particularly to a touch sensing circuit and method capable of detecting an interference signal and adjusting charging and detection periods of the sensing circuit according to a detection result to enhance the sensing accuracy.
Description of the Related Art
FIG. 1 shows a function block of a conventional touch sensing circuit 100. The touch sensing circuit 100 includes a sensing device, a sensed value analyzing circuit 120, a control signal generating circuit 130 and a determining unit 140. For a capacitive touch panel, many transparent electrodes are disposed below a surface glass cover layer, and capacitance devices are equivalently formed between the transparent electrodes and a substrate of the touch panel. In FIG. 1, a plurality of sensing units 112 included in the sensing device 110 correspond to these capacitance devices. When a touch event occurs above the glass cover layer, equivalent capacitance between the transparent electrodes and the human body causes changes in capacitance values of the sensing units 112. That is, a sensed value generated by the sensing device 110 is changed due to the touch event. The sensed value analyzing circuit 120 analyzes the sensed value according to a control signal generated by the control signal generating circuit 130 and generates a sensing signal. According to the sensing signal, the determining unit 140 then determines information such as a position, the number of times and the duration of a touch position.
FIG. 2 shows a detailed circuit diagram of a conventional sensed value analyzing circuit 120. The sensed value analyzing circuit 120 includes switches 121, 122 and 123, operational amplifiers 124 and 126, and a capacitor 125. The switches 121, 122 and 123 are controlled by a control signal CTRL of the control signal generating unit 130 to be periodically turned on and turned off. The sensed value analyzing circuit 120 charges and detects (or samples) the sensing unit 112 according to different conduction paths. In a charging phase, the control signal CTRL turns on the switches 121 and 123 and turns off the switch 122, and an output end of the operational amplifier 124 charges the sensing unit 112 by a voltage Vref1, while the capacitor 125 performs a discharging process. At the end of the charging phase, the end voltage of the sensing unit 112 is charged to Vref1 and charges in the capacitor 125 are fully discharged. In a subsequent detection phase, the control signal CTRL turns off the switches 121 and 123 and turns on the switch 122. At this point, the charges on the sensing unit 112 are redistributed to the sensing unit 112 and the capacitor 125. As the capacitance value of the capacitor 125 and voltages Vref1 and Vref2 are set in advance, the determining unit 140 can obtain touch control information through detecting the voltage change of the output end of the operational amplifier 126 and accordingly determine the capacitance value (i.e., the sensed value) of the sensing unit 112.
In practice, a detection result of a touch panel may be incorrect due to the interference of external signals. For example, a charger with poor quality may couple noises to a touch panel, whose reference level may then fluctuate according to the noises. For the touch panel, the noises are equivalently coupled to the sensing nit 112 through the human body. FIG. 3 shows a circuit diagram of a conventional touch sensing circuit accompanied with a noise source. A noise source 310 is coupled to the touch panel via a contact node between the sensing unit 112 and the ground, and affects the charging and detection processes that the sensed value analyzing circuit 120 performs on the sensing unit 112. FIG. 4 shows a relationship diagram of the charging phase and the detection phase of the conventional sensed value analyzing circuit 120 corresponding to an interference signal. According to the control signal CTRL of the control signal generating unit 130, the sensed value analyzing circuit 120 performs a charging process on the sensing unit 112 in a charging phase P and performs a detection process on the sensing unit 112 in a detection phase S. The charging phase P and the detection phase S occur alternately, and have the same time period (5 μs in this example). A curve 410 is an interference signal of the noise source 310, and contains drastic changes in a range selected by the dotted frame. When the interference signal changes drastically at the instant when the charging phase or the detection phase S ends, the sensing unit 112 cannot be charged to the predetermined voltage Vref1, and a final detection result is affected by the distribution of charges on the sensing unit 112 and the capacitor 125 in the detection phase S.